Keratomycosis - Fungal Infection of the Eye

Fungal keratitis is a serious and painful corneal disease that is caused by fungal infection.

The most common fungal species isolated from the normal equine eye are Aspergillus, Penicillium, Alternaria, Cladosporium, and Fusarium. Corneal injuries in horses often are caused by exposure to mold contaminated buildings which have been more prevalent in the past twenty years due to current weather trends and shoddy construction work/cheap building materials.

The clinical appearance of fungal keratitis varies greatly depending on the duration and severity of infection. It may present as either an ulcerative or non-ulcerative corneal lesion. Ulcerative fungal keratitis may appear as a distinct lesion with a uniform white or yellow border and base; with a necrotic, dry, shaggy raised border; as a gray or white edematous necrotic central corneal lesion; or as multiple white or gray punctuate lesions of the epithelium and superficial stroma. Occasionally fungal keratitis will appear as an abscess within the corneal stroma. These abscesses may be located very deep in the stroma and close to Descemet’s membrane. It is possible for fungal organisms to penetrate Descemet’s membrane and gain access to the anterior chamber. Deep corneal invasion of the fungi and concurrent bacterial infection can lead to corneal perforation and iris prolapse. Common clinical signs of fungal keratitis include ocular pain manifested by blepharospasm, epiphora and/or photophobia, fluorescein-positive corneal ulceration, corneal neovascularization, and uveitis manifested by miosis and aqueous flare.

Diagnosis is based on the history and clinical signs, with confirmation from cytology and/or culture results. Corneal scrapings should be obtained for cytology and culture in all cases of suspected fungal keratitis. Samples can be collected with a sterile Kimura spatula or the sharp or dull side of a sterile No. 15 scalpel blade. It has been found that micro-brushes are also useful for collecting cytology samples. If a corneal stromal abscess is present, deep scraping is needed to obtain good samples. If the abscess is very deep, sample collection should be postponed until surgical treatment is possible and the sample should be obtained at that time. Both ulcerated and subepithelial lesions should be debrided to facilitate the collection of superficial stromal tissue for diagnosis and enhance drug penetration during treatment. Scraping samples should be placed on a glass slide and air-dried, then stained with Gram’s, Wright’s, or Giemsa stains. Culture specimens should be immediately inoculated onto Sabouraud’s agar or transferred onto a sterile swab to be inoculated into a thioglycollate broth. The most common reason of failing to obtain a positive result is poor sample collection (a superficial scraping). Aspergillus and Fusarium are the most commonly isolated fungi.

Dr. J. Tobin